Sir Hans Adolf Krebs ( or ) (25 August 1900 – 22 November 1981) was a German-born British physician and biochemist. He was the pioneer scientist in study of cellular respiration, a biochemical pathway in cells for production of energy. He is best known for his discoveries of two important chemical reactions in the body, namely the urea cycle and the citric acid cycle. The latter, the key sequence of metabolic reactions that produces energy in cells, often eponymously known as the "Krebs cycle", earned him a Nobel Prize in Physiology or Medicine in 1953. With Hans Kornberg, he also discovered the glyoxylate cycle, which is a slight variation of the citric acid cycle found in plants, bacteria, protists, and fungi.